assert() with message

I saw somewhere assert used with a message in the following way:

assert(("message", condition));

This seems to work great, except that gcc throws the following warning:

warning: left-hand operand of comma expression has no effect

How can I stop the warning?

Use -Wno-unused-value to stop the warning; (the option -Wall includes -Wunused-value).

I think even better is to use another method, like

assert(condition && "message");

Try:

#define assert__(x) for ( ; !(x) ; assert(x) )

use as such:

assert__(x) {
    printf("assertion will fail\n"); 
}

Will execute the block only when assert fails.

> IMPORTANT NOTE: This method will evaluate expression x twice, in case x evaluates to false! (First time, when the for loop is checking its condition; second time, when the assert is evaluating the passed expression!)

If you want to pass a formatted message, you could use the following macros:

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <assert.h>

#define clean_errno() (errno == 0 ? "None" : strerror(errno))
#define log_error(M, ...) fprintf(stderr, "[ERROR] (%s:%d: errno: %s) " M "\n", __FILE__, __LINE__, clean_errno(), ##__VA_ARGS__)
#define assertf(A, M, ...) if(!(A)) {log_error(M, ##__VA_ARGS__); assert(A); }

Then use it like printf:

// With no args
assertf(self != NULL,"[Server] Failed to create server.");

// With formatting args
assertf((self->socket = u_open(self->port)) != -1,"[Server] Failed to bind to port %i:",self->port);
// etc...

Output:

> [ERROR] (../src/webserver.c:180: errno: Address already in use) > [Server] Failed to bind to port 8080: webserver: > ../src/webserver.c:180: server_run: Assertion `(self->socket = > u_open(self->port)) != -1' failed.

Based on http://c.learncodethehardway.org/book/ex20.html

By tradition, (void) communicates to the compiler that you are knowingly ignoring an expression:

/* picard.c, TNG S6E11. */
#define assertmsg(x, msg) assert(((void) msg, x))
assertmsg(2+2==5, "There! are! four! lights!");

A function that takes const char* and returns true would probably save you from all sorts of warnings:

#include <assert.h>

int always_true(const char *msg) {
    return 1;
}

#define assert_msg(expr, msg) assert((expr) && always_true(msg))

For unexpected default case of a switch, an options is

assert(!"message");

In my case, I changed @pmg's answer to be able to control the output. The (... && "message") didn't work for me.

#include <assert.h>
#include <stdio.h>

#define __DEBUG__ 1

assert ((1 == 1) && 
       (__DEBUG__ && printf("  - debug: check, ok.\n")) || !__DEBUG__);

You could write your own macro that provides the same usage of _Static_assert(expr, msg):

#include <assert.h>
#include <stdbool.h>
#include <stdio.h>


/*
 * void	assert_msg(bool expr, const char *msg);
 */
#if !defined(NDEBUG)
#define assert_msg(expr, msg)	do					\
{													\
		const bool	e_ = expr;						\
													\
		if (!e_) {									\
				fputs(msg, stderr);					\
				fputc('\n', stderr);				\
				assert(e_);							\
		}											\
} while (0)
#else
#define assert_msg(expr, msg)	do					\
{													\
													\
		if (!(expr))								\
				warn_bug(msg);						\
} while (0)
#endif

I also have a macro warn_bug() that prints the name of the program, the file, the line, the function, the errno value and string, and a user message, even if asserts are disabled. The reason behind it is that it won't break the program, but it will warn that a bug will probably be present. You could just define assert_msg to be empty if defined(NDEBUG), though.

According to following link http://www.cplusplus.com/reference/clibrary/cassert/assert/

assert is expecting only expression. May be you are using some overloaded function.

According to this, only expression is allowed and thus you are getting this warning.